Efficient cation recognition strategies for cationic compounds

(Press-News.org) Huang Feihe at Zhejiang University, Jonathan Sessler of the University of Texas at Austin, and colleagues reported a novel cation recognition mode which mimics the biological allosteric effect and achieves efficient recognition of cations by cationic compounds. Specifically, this work achieves continuous recognition of anions and cations by synergizing various recognition modes while also utilizing the allosteric effect during the recognition process to explore a new cation recognition mode.

Background:

Coulomb's law, one of the fundamental laws of physics, explains the electrostatic interaction principle that like charges repel and unlike charges attract. Within this framework, traditional cation recognition systems primarily rely on anionic and neutral host molecules, making it difficult for cationic compounds to effectively recognize cations. However, if this recognition limitation could be overcome and a cation recognition system based on a cationic host molecule could be developed, it would enrich supramolecular recognition modes, expand the scope of cation recognition, and open up broader application prospects for cation recognition in materials and biological sciences.

Highlights of this article :

In response to this scientific question and challenge, we successfully developed a class of cationic capsule molecules. First, we used these cationic capsule molecules to recognize anions. During anion recognition, the capsule molecules undergo conformational changes. This dynamic structural adjustment enhances their ability to recognize cations, thereby achieving highly efficient cation recognition.

Building on previous research, we first successfully prepared two counterion forms of the capsule molecule: [ 3NDI 6+ @2PF 6 − ]·4PF 6 − and [ 3NDI 6+ ⊃ 2Cl − ]·4Cl − . We then comprehensively characterized these two forms of the capsule molecule. Interestingly, our study found that the nuclear magnetic resonance hydrogen spectra ( 1 H NMR) of these two counterion forms showed significant differences, and their ultraviolet-visible spectra (UV-Vis spectra) and cyclic voltammetry (CV) curves also showed obvious differences. This phenomenon aroused great interest because, under normal circumstances, counterions (such as PF 6 − and Cl − ) do not cause changes in the nuclear magnetic resonance hydrogen spectrum. In addition, through variable temperature nuclear magnetic resonance hydrogen spectroscopy (VT NMR) analysis, we found that the spectra of these two forms exhibited completely different spectral characteristics under temperature changes, indicating that there are significant differences in their molecular symmetry.

We speculate that the spectral differences arise from the formation of distinct host-guest inclusion complexes between the two counterion forms and the capsule molecular backbone, resulting in distinct spatial conformations of the capsule molecule. This speculation was confirmed by single crystal X-ray analyses. The single crystal structures clearly reveal that the two counterion forms of the capsule molecule possess distinct structural features: in the [ 3NDI 6+ @2PF 6 − ]·4PF 6 − molecule , the two PF 6 − groups are located in the capsule molecule's external cavity; whereas in the [ 3NDI 6+ ⊃ 2Cl − ]·4Cl − molecule , the two Cl − groups occupy the capsule molecule's internal cavity. This distinct recognition pattern directly leads to the significant differences in the capsule molecule's shape. To further explore the anion recognition ability of the capsule molecule [ 3NDI 6+ @2PF 6 − ] 4+ molecule, we performed quantitative analysis using isothermal titration calorimetry (ITC). The experimental results show that compared with PF 6 − , Cl − exhibits stronger binding ability with the capsule molecules, and their binding constants are Ka ,1  = (1.98 ± 0.01) × 10 5  M −1 and Ka ,2 = (7.40 ± 0.60) × 10 3  M −1 , respectively.

Based on the above, we draw the following conclusions: in the process of recognizing Cl −, the molecular conformation of the capsule molecule [3NDI 6+ @2PF 6 − ]·4PF 6 − undergoes a significant transformation, from a collapsed type to a prismatic type, thus forming a cavity inside the capsule molecule.

Because anion recognition creates an internal cavity, we further investigated its ability to recognize cations. Our results demonstrate that the capsule, after undergoing anion recognition, can efficiently recognize cations, thereby effectively enabling cationic compounds to effectively recognize cations. We attribute this successful recognition to two key factors: first, the allosteric effect creates an effective cavity that provides the necessary spatial conditions for cation recognition; and second, the Cl − within the cavity effectively attracts the cation through electrostatic interactions.

Summary and Outlook:

In summary, this paper reports a strategy for achieving effective cation recognition by cationic compounds through allosteric effects, demonstrating significant potential for applications in cation-recognition-based materials, molecular machines, and biosensors. The research results were recently published in CCS Chemistry, the flagship journal of the Chinese Chemical Society. The first authors are Dr. Fang Shuai of the University of Hong Kong, Dr. Wang Mengbin of Zhejiang University, and Dr. Tang Chun of the University of Hong Kong. Corresponding authors are Professors Huang Feihe of Zhejiang University and Jonathan Sessler of the University of Texas at Austin. This research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Zhejiang Province, and the Department of Chemistry at the University of Hong Kong.

---

About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.

END